write externals module to externals.txt (#2363)

Author: yowl

src/coreclr/tools/aot/ILCompiler.LLVM/CodeGen/LLVMObjectWriter.cs

@@ -282,7 +282,7 @@ private void FinishObjWriter()
             _module.PrintToFile(Path.ChangeExtension(_objectFilePath, ".txt"));
             _module.Verify(LLVMVerifierFailureAction.LLVMAbortProcessAction);
 
-            _module.PrintToFile(Path.ChangeExtension(_objectFilePath, "external.txt"));
+            _moduleWithExternalFunctions.PrintToFile(Path.ChangeExtension(_objectFilePath, "external.txt"));
             _moduleWithExternalFunctions.Verify(LLVMVerifierFailureAction.LLVMAbortProcessAction);
 #endif